Fluid pressure actuated pivoted clamp arms are known in the art, for example, such as illustrated in U.S. Pat. No. 5,115,110 wherein an hydraulically actuated spring release base mounted clamp is disclosed. In such case, a central short stroke hydraulic cylinder produces pivotal clamp engagement of dual laterally spaced clamp arms, the pivots of which can be laterally retracted to clearance position upon release of hydraulic pressure. A cross-shaft having a flatted engagement by the rigidly mounted cylinder piston head and a pivot cross-shaft having flatted ends adapted to slide in slotted keyways upon release of actuating pressure provide lateral movement of the clamp arm assembly to a clearance position while the actuating cylinder remains in a fixed position.